The present invention relates to a double screw rotor assembly, and more particularly to such a double screw rotor assembly which is suitable for use in a fluid pressure changer means, such as water or air pumps, compressors, etc.
FIGS. 4 and 5 show a traditional double screw rotor assembly. This structure of double screw rotor comprises a casing 5 defining a compression chamber 51, two screw rotors 6 meshed inside the compression chamber 51. Because the thread 60 of each rotor 6 has constant pitch P and same cross-sectional profile, the volume of each transfer chamber 52 does not vary with the operation of the two screw rotors 6. Because the volume of transfer chamber 52 is not variable, a high pressure difference occurs at the outlet end 512 during the operation of the screw rotors 6, and this high pressure difference results in a reverse flow of fluid, high noises, and waste of energy.
U.S. Pat. No. 5,667,370 discloses another structure of double screw rotor assembly as shown in FIG. 6. According to this design, the thread 61 of each screw rotor 60 has an uneven pitch P.sub.1,P.sub.2, and the pitch P.sub.1,P.sub.2 is reduced in proper order from the inlet end 513 toward the outlet end 514 (P.sub.1 &gt;P.sub.2), therefore the volume of respective chamber 53 or 54 are gradually reduced in same direction. However, the non-uniform pitch type of thread 61 defines different cross-sectional profiles and pressure angles at different elevations. These limitations cause the thread 61 difficult to be produced. When processing the thread 61, a particularly designed cutting-metal working machine must be used. This complicated thread processing process greatly increases the manufacturing cost of the double screw rotor assembly.